Bleaching of chemical pulps is mainly carried out with chlorine-containing bleaching agents, such as chlorine, chlorine dioxide and hypochlorite, resulting in chloride-containing, corrosive spent bleach liquors which therefore are difficult to recover and thus results in detrimental discharges to the environment. Nowadays, there is a strive towards the use of, to the greatest possible extent, bleaching agents poor in or free from chlorine, so as to reduce the discharges and recover the spent liquors. One example of such a bleaching agent, which recently has come into increasing use, is oxygen. By using an initial alkaline oxygen stage in a multi-stage bleaching sequence of, for example, sulphate pulp, it is possible to reduce the discharge from bleach plants by more than half the original amount, since spent oxygen bleach liquor not containing chlorine is recoverable. However, after an initial oxygen bleaching stage, the remaining lignin left in the pulp is about half of the amount remaining after the delignification in the cooking process, which thus has to be dissolved out of the pulp by further bleaching by means of chlorine-containing bleaching agents. Therefore, there is a tendency to further reduce, by means of various pretreatments and prebleaching stages, the amount of lignin that has to be removed by chlorine-containing bleaching.
Other types of bleaching chemicals which are suitable from a recovery point of view, include peroxides, e.g. inorganic peroxides, such as hydrogen peroxide and sodium peroxide, and organic peroxides, such as peracetic acid. In actual practice, hydrogen peroxide is not used to any appreciable extent in the first step of a bleaching sequence to obtain an initial reduction of lignin and/or an increase in brightness, because of the large amounts of added hydrogen peroxide which are necessary.
Thus, large amounts of hydrogen peroxide must be added in alkaline hydrogen peroxide treatment to reach a satisfactory dissolution of lignin, since such a treatment gives a high degree of decomposition of the hydrogen peroxide, resulting in considerable costs for chemicals. In acidic hydrogen peroxide treatment, the same dissolution of lignin can be obtained as in alkaline treatment with a much lower consumption of hydrogen peroxide. However, the acidic treatment results in a substantial drop in the viscosity of the pulp, i.e. the decomposition products of the hydrogen peroxide, at low pH values attack not only the lignin, but also the cellulose, so that the length of the carbohydrate chains is reduced, resulting in impaired strength properties of the pulp. Furthermore, an intensely acidic treatment is inconvenient since it involves the precipitation of lignin already dissolved, the resin becomes sticky and difficult to dissolve, and problems arise regarding the recovery of the acidic spent liquor.
According to SE-A 420,430, the drop in the viscosity in an acidic hydrogen peroxide treatment can be avoided by carrying it out in the presence of a complexing agent, such as DTPA (diethylenetriaminepentaacetic acid), at a pH of from 0.5 to 3.0. This treatment step is followed by an alkaline extraction step for removal of dissolved lignin, without intermediate washing.
Furthermore, it is known to remove trace metals from cellulose pulps by using the combined effects of sodium sulphite (SO.sub.2 in an alkaline solution) and DTPA before the peroxide treatment step, see Gellerstedt et al, Journal of Wood Chemistry and Technology, 2(3), 231-250 (1982). By this, complexes of DTPA and a reduced metal ion are formed and which can be removed from the pulp by washing, whereupon a hydrogen peroxide treatment with improved efficiency can be carried out.
For mechanical pulps, it is common practice to include pretreatment with complexing agents in a bleaching sequence, prior to an alkaline hydrogen peroxide stage, see e.g. EP 285,530, U.S. Pat. No. 3,251,731 and SU 903,429. In this case, however, the aim is purely to bleach the pulp and not to delignify it. For this purpose, the activity of hydrogen peroxide is controlled by the addition of silicates, such as sodium silicate, so that on the whole it is the content of chromophoric groups which is reduced. Failure to include silicate in the bleaching composition will prevent the mechanical pulp from gaining the best obtainable brightness, even if the charge of hydrogen peroxide is substantially increased, e.g. by 50% above the normally added quantity. For chemical pulps, the addition of silicates is avoided, since this would only increase the cost for chemicals without any positive effect and make it impossible to easily recover the waste liquors. Furthermore, for chemical pulps the increase in brightness is definitely influenced by a change of pH in the complexing stage, whereas this is not the case when treating mechanical pulps with complexing agents.